This invention relates generally to an electric shift apparatus especially suited for use with a motor vehicle having an automatic transmission and, more particularly, to a manual override for such an electric shift apparatus.
Motor vehicles since their inception have required some manner of gear change mechanism to satisfy the varying torque and speed requirements encountered during the typical duty cycle of a motor vehicle. For many years these gear change mechanisms were manual in the sense that they required an operator input from a shift lever or the like to effect each desired gear change ratio. More recently, so-called "automatic" transmissions have become popular in which much of the shifting is done without operator input in response to sensed speed and throttle opening parameters. These automatic transmissions typically include a mode select member on the transmission housing movable between a plurality of selected positions corresponding to a respective plurality of shift modes within the transmission. The mode select member is moved between its several shift positions by a cable or linkage mechanism extending from the mode select member to a suitable gear selector lever located in the passenger compartment of the vehicle. Various proposals have been made in the past to eliminate the mechanical interconnection between the driver operated lever and the mode select member and provide instead an electric shift apparatus in which electrical signals generated by a suitable action on the part of the driver are transmitted electrically to some manner of power means arranged to move the mode select member.
In one such proposal disclosed in U.S. patent application Ser. No. 095,856, assigned to the assignee of the present invention, the electric shift apparatus includes an electric motor having a housing and an output shaft; a speed reduction unit having a housing secured to the motor housing and having an output shaft driven at a reduced speed in response to rotation of the motor output shaft and having a free end positioned outside of the speed reduction unit housing; means for mounting the motor and speed reduction unit on the transmission housing with the output shaft of the speed reduction unit aligned coaxially with the mode select shaft of the transmission and the free ends of the shafts juxtaposed; coacting coupling means on the free ends of the shafts operative to drive the mode select shaft in response to rotation of the reduction unit output shaft; and encoder means operative to sense the shift position of the transmission and generate an encoder signal representative of the sensed shift position. Whereas this arrangement provides a compact and efficient shift apparatus which functions in most situations to effectively drive the mode select shaft of the transmission between its various shift positions, there are certain emergency situations in which this arrangement is not totally satisfactory. Specifically, if a total electrical failure is encountered while driving the associated vehicle, the vehicle will stop with the transmission in its drive position. If an attempt is thereafter made to jump-start the vehicle, the attempt will be unsuccessful since the vehicle will only start with the transmission in its neutral position or its park position. Further, if the vehicle is parked in a confined space such as a garage with the transmission in the park position and the vehicle thereafter cannot be started due to a battery failure or the like, the vehicle is difficult to start or move since the confined space of the garage makes jumping difficult and the park disposition of the transmission makes towing impossible.